The job of the print-Y-axis function is to print a label for
the vertical axis that looks like this:

10 -
5 -
1 -

The function should be passed the height of the graph, and then should
construct and insert the appropriate numbers and marks.

It is easy enough to see in the figure what the Y axis label should
look like; but to say in words, and then to write a function
definition to do the job is another matter. It is not quite true to
say that we want a number and a tick every five lines: there are only
three lines between the `1' and the `5' (lines 2, 3, and 4),
but four lines between the `5' and the `10' (lines 6, 7, 8,
and 9). It is better to say that we want a number and a tick mark on
the base line (number 1) and then that we want a number and a tick on
the fifth line from the bottom and on every line that is a multiple of
five.

The next issue is what height the label should be. Suppose the maximum
height of tallest column of the graph is seven. Should the highest
label on the Y axis be `5 -', and should the graph stick up above
the label? Or should the highest label be `7 -', and mark the peak
of the graph? Or should the highest label be 10 - , which is a
multiple of five, and be higher than the topmost value of the graph?

The latter form is preferred. Most graphs are drawn within rectangles
whose sides are an integral number of steps long--5, 10, 15, and so
on for a step distance of five. But as soon as we decide to use a
step height for the vertical axis, we discover that the simple
expression in the varlist for computing the height is wrong. The
expression is (apply 'max numbers-list) . This returns the
precise height, not the maximum height plus whatever is necessary to
round up to the nearest multiple of five. A more complex expression
is required.

As usual in cases like this, a complex problem becomes simpler if it is
divided into several smaller problems.

First, consider the case when the highest value of the graph is an
integral multiple of five--when it is 5, 10, 15 ,or some higher
multiple of five. In this case, we can use this value as the Y axis
height.

A fairly simply way to determine whether a number is a multiple of
five is to divide it by five and see if the division results in a
remainder. If there is no remainder, the number is a multiple of
five. Thus, seven divided by five has a remainder of two, and seven
is not an integral multiple of five. Put in slightly different
language, more reminiscent of the classroom, five goes into seven
once, with a remainder of two. However, five goes into ten twice,
with no remainder: ten is an integral multiple of five.